1. Field of the Invention
This invention relates generally to diesel engines and, more particularly, to medium speed diesel engines for operation at high altitudes.
2. Background Art
Power is generated in a diesel engine by diffusing and combusting diesel fuel in a plurality of engine cylinders. Liquid fuel is injected into the engine cylinders, which are full of compressed air at high temperature. The fuel is broken up into droplets, which evaporate and mix with the air in the cylinders to form a flammable mixture. Complete and efficient combustion in the cylinders requires full oxidation of fuel through evaporation, species diffusion, and mixing with air, and timely heat release during the combustion process. Thus, the amount of air charged into the cylinder, or the air to fuel ratio of the mixture, plays an important role in diesel engine fuel-air mixing and combustion, which, in turn affects fuel efficiency and exhaust emissions. This is particularly true for quiescent chamber type medium speed heavy-duty diesel engines where the cylinder air intake swirling is slight, such as locomotive or marine engines having cylinders with relatively large displacement volumes. The fuel injection timing of medium speed diesel engines operating at full load is typically set so that the actual peak firing pressure in the cylinders is at or below a maximum cylinder firing pressure under normal altitude operation, i.e., at about sea level.
Engine exhaust emissions, including carbon monoxide (CO), particulate matter (PM), and smoke, are generated when the air-fuel mixture is incompletely combusted. When engines are operated at higher altitudes, i.e., at a low barometric pressure, lesser amounts of air are introduced into the cylinders, causing the air-fuel mixing process to be deteriorated relative to lower altitude, higher ambient pressure environments. This combination of factors increases late and incomplete combustion in the engine cylinders, which lowers fuel efficiency and increases exhaust emissions of CO, PM, and smoke. The reduced amount of air for the fuel-air mixture combustion, together with the increased amount of late and incomplete combustion, typically leads to reduced peak cylinder firing pressure and increased cylinder exhaust gas temperatures. For engines including a turbocharger, the decreased barometric pressure and the increased exhaust temperature cause an increase in turbocharger speed. This usually requires power deration to prevent turbocharger damage from overheating and excessive speed.
Accordingly, it would be desirable to operate a diesel engine at higher altitudes that avoids the resultant increase in exhaust emissions. Additionally, it would be desirable to operate a diesel engine at higher altitudes with minimal deterioration of engine efficiency, power capacity, and engine performance relative to normal altitude operation.